Preparation of norbornene or monoalkyl substituted norbornenes is well known in the art. Normally an .alpha.olefin is condensed with cyclopentadiene or dicyclopentadiene at elevated temperatures and pressures. However, the basic process is unsatisfactory because considerable amounts of resinous materials are formed which slowly build up in the reactor and associated equipment. The internally unsaturated olefins are somewhat less reactive than .alpha.olefins which require even higher reaction temperatures and longer residence time which causes even greater buildup of thermal polymers than when .alpha.-olefins are employed. To combat the polymer buildup prior art has suggested carrying out the reaction in the presence of a large excess of olefin (U.S. Pat. No. 2,991,308), or in the presence of hydrocarbon solvent (U.S. Pat. No. 3,766,283). Although hydrocarbon solvents are helpful in reducing the polymer buildup when norbornene or monoalkyl norbornenes are prepared, there is still a considerable polymer buildup which makes the process not very practical commercially. However, in the preparation of dialkylnorbornenes or cyclic norbornene derivatives hydrocarbon solvents have little or no effect on the buildup of polymers.